The invention relates to the deposition of thin films on flexible substrate members such as plastic sheeting.
Known methods and structures for coating substrates are not economical or efficient to the extent that the invention herein teaches. Such known methods and structures have resulted in nonuniform and expensive product.
Generally a plasma vapor condition is established in a vacuum or pressure vessel and the substrate members are exposed to the vapor in batches. The plasma vapor condition is established between a member known as a target and a substrate mounting member. The atmosphere in the vessel contains some noble gas which is ionized and the ions bombard the target to drive atoms of the material out of the target to the substrate holding means. In the case of electrically established plasma vapor, the target is a cathode and the substrate holding means comprise the anode. Both are cooled. The material to be deposited is either conductive or nonconductive. In the case of nonconductive material, the plasma vapor must be established between the target and anode by means of radio frequency. In the case of conductive materials being deposited, the field may be either d.c. or r.f.
Known methods and apparatus for coating by this general technique have been characterized by low throughput principally caused by the construction of the apparatus in such a manner as not to be able to handle substrate at high rates of speed and in such a manner as not to deposit material uniformly and easily.
The most advantageous method of depositing is one in which the relationship between the projecting unit target area and the unit exposed area of the substrate is greater than one to one. Most commercial depositing apparatus and methods do not take advantage of this method and in fact have a relationship which is less than one to one. Typical are those structures in which a central target rod is fixed at the center of a coaxial rotating anode cage with small members of substrate exposed on the interior surface of the cage. In this latter type of structure it can be appreciated that the plasma vapor cloud thins as it approaches the substrate members. One to one relationship arrangements occur in laboratory or small batch operations only, so far as is known, where the target is a planar plate and the substrate is arranged parallel with the plate. The plasma vapor drives the particles of the target material to the substrate member principally in approximately parallel paths.
Some work has been done using the advantageous arrangement which is inherently capable of depositing at high rates, but such work has been deficient in many respects. So far as known, none of it has resulted in any commercially feasible sputtering. The basic concept is that of providing a central substrate-carrying drum which rotates within a cylindrical target so that the ratio of projecting unit target area to unit exposed area of the substrate is substantially greater than unity. Structures which disclose this concept are found in U.S. Pat. Nos. 3,282,816; 3,528,902; 3,314,873. In U.S. Pat. No. 3,314,873 the articles being coated pass through near the center of a cylindrical target without rotating, but moving lengthwise.
According to the invention as described in detail herein, specific examples of which are set forth in application Ser. No. 323,133, practical apparatus and methods are disclosed which enable maximum advantage to be taken of the technique in which the above-mentioned ratio is substantially more than unity.
Before further discussing the problems which have been solved and describing the advantages of the invention, it would be helpful to define some of the terms and expressions to be used hereinafter.